1. Field of the Invention
The present invention relates to a ceramic catalyst body preferably applied, for example, as a catalyst for purifying the exhaust gas of automobile engines.
2. Description of the Related Art
Typical three-way catalysts used as catalysts for purifying exhaust gas normally have a structure in which a coating layer of γ-alumina and so forth is formed on the surface of a ceramic support composed of highly thermal shock-resistant cordierite honeycomb structure followed by the loading of a precious metal catalyst. However, since problems such as increased thermal capacity and increased pressure loss occur due to the formation of the coating layer, studies have been conducted on the loading of catalyst components without forming a coating layer. Although Japanese Examined Patent Publication No. 5-50338 describes a method for increasing the specific surface area of the cordierite itself by heat treatment following acid treatment, the resulting destruction of the crystal lattice of the cordierite caused by the acid treatment and heat treatment results concerns over decreased strength, thereby leading to problems in terms of practicality.
Consequently, the inventors of the present invention previously proposed a ceramic support that enables catalyst components to be loaded directly into fine pores formed by substituting at least one type of element that composes the base ceramic with an element having a different valency (Japanese Unexamined Patent Publication No. 2001-310128). Since this ceramic support does not require a coating layer for improving specific surface area and eliminates the problem of decreased strength caused by acid treatment and so forth, it is expected to be applied to automobile catalysts requiring durability.
In addition, various co-catalyst components have been added to improve catalyst performance, and in the case of three-way catalysts, a co-catalyst component such as ceria is used that has the ability to occlude oxygen. Ceria has the characteristic of occluding and releasing oxygen in the vicinity of the theoretical air-fuel ratio, and the range (window) over which high purification performance can be obtained can be widened by taking advantage of this characteristic. Therefore, even in a composition in which the previously mentioned ceramic support is used that enables catalyst components to be loaded directly, attempts have been made to improve purification performance by loading a precious metal catalyst followed by loading a co-catalyst component thereon.
On the other hand, accompanying implementation of increasingly strict regulations on automobile exhaust gas, it is becoming necessary to increase the loaded amounts of precious metal catalyst and co-catalyst components in order to obtain the desired catalyst performance. However, as the amount of co-catalyst components increases, the distance between these components and the precious metal catalyst on the support surface increases, thereby resulting in the problem of decreased performance. In addition, pressure loss increases as a result of the increasing thickness of the co-catalyst layer formed on the cell wall surfaces. In particular, the wall thickness of ceramic supports has tended to be reduced in recent years for the purpose of reducing thermal capacity, while the intervals between cells have been shortened to ensure support strength, and the loading of co-catalyst components has a remarkable effect on pressure loss. Consequently, there is a need to maximally demonstrate the effects of co-catalyst components while using them in the lowest possible amounts.
In consideration of the circumstances described above, an object of the present invention is to provide a ceramic catalyst body offering low pressure loss and high purification performance that inhibits increases in pressure loss and improves catalyst performance in a composition in which a primary catalyst component and co-catalyst component are loaded on a direct-loading catalyst having low thermal capacity and high strength.